1. Field of the Invention
The present invention relates to battery monitoring systems capable of monitoring a state of each of the unit cells forming a battery pack.
2. Description of the Related Art
In general, a battery pack and an electric motor are mounted as a main vehicle engine on vehicles such as electric vehicles and hybrid vehicles. Such a battery pack is a high capacity pack capable of supplying a high voltage to the electric motor. The battery pack consists of a plurality of unit cells made of lithium ion rechargeable batteries, etc. The unit cells are connected in series.
In general, because each of the unit cells connected in series to form the battery pack has different characteristics, each unit cell has a different stage of charge (SOC) when the battery pack is charged and discharged. Accordingly, there is a possible problem of causing deterioration of a unit cell which enters an overcharge or over discharge state.
In order to avoid such a problem, a conventional technique detects a dispersion of SOC between unit cells in the battery pack in order to equalize the voltage of each of the unit cells. That is, the conventional technique detects an analogue voltage of each of the unit cells connected in series in the battery pack, and an analogue digital converter (ADC) performs an analogue to digital conversion (AD conversion) to convert the detected analogue voltage to a digital voltage. The conventional technique uses the obtained digital voltage as the voltage of each unit cell. The conventional technique calculates a difference in digital voltage between the unit cells, and uses the calculated voltage difference as a voltage dispersion in the unit cells. For example, a patent document 1, Japanese patent laid open publication No. 2009-276297 discloses such a conventional technique.
The ADC has an input voltage range which corresponds to a possible voltage range of each unit cell. When receiving a voltage (i.e. a voltage difference between unit cells), the ADC performs the AD conversion of the received analogue voltage to digital voltage values having a predetermined bit number).
A voltage conversion error occurs when the ADC performs the AD conversion. The more the AD conversion error of the AD conversion increases, the more the influence on a calculation accuracy of the voltage difference between the unit cells increases. For this reason, it is necessary to decrease the AD conversion error in order to increase the calculation accuracy of the voltage difference between the unit cells forming the battery pack.